The invention relates generally to nondestructive methods for measuring stress in ferromagnetic material caused by deformed regions or pressure exerted on the surface of the material. More particularly, the invention is a system and method that relies on the Barkhausen effect to measure properties in regions of ferromagnetic materials that are subjected to various types of stress. While most applications of the Barkhausen effect rely on a time-varying magnetic field to excite the magnetic domains that generate Barkhausen noise in a ferromagnetic material, the present invention relies on a moving, steady state magnetic field generated by a permanent magnet or electromagnet excited by a DC current to create Barkhausen magnetic transitions.
Measurement and characterization of stress and material loss due to erosion, corrosion and gouging in pipelines are critical to early detection of impending failure in order to prevent a situation that is dangerous or destructive to personnel, wildlife or the environment. These concerns have created an increased need to detect and measure anomalous regions of pipe wall stress and strain. Pipelines are subjected to continuous stress from the pressure maintained within the pipeline required to move the pipeline contents through the pipeline. Some pipelines are also subject to ground settlement or movement that may put the pipe wall in a high-stress condition. Bends in a pipe also create stress that varies around the circumference of the pipe. Mechanically damaged areas, such as dents and gouges, have been shown to contain plastically deformed zones, and detection of these zones is a means for identifying mechanical damage. These mechanical defects create residual stress on the pipe inner surface. Once pipelines are installed, they are expected to provide safe and reliable operation over several decades. In addition, inspection and detection of metal loss and stress in long pipelines is more difficult where the pipelines are buried underground or are positioned on an ocean floor.
The most widely used method for in-line inspection and measurement of internal and external material loss in a pipeline wall is magnetic flux leakage (MFL) detection implemented in an inspection pig. Pumping an electronically instrumented MFL inspection pig through a pipeline from one compressor station to the next provides for in-line inspection of the pipeline. The MFL inspection pig may contain a circumferential array of MFL detectors embodying strong permanent or DC electromagnets to magnetize the pipe wall to near saturation flux density. As the inspection pig moves through the pipeline, metal loss, such as corrosion pits, cause an increase in magnetic flux density outside of the pipe material near the corrosion pits that may be detected by Hall effect sensors or induction coils. While the MFL inspection pig has proved effective in detecting surface defects, it lacks an effective means for determining stress in pipe walls. Several other technologies have been developed to determine stress in steel parts such as pipe walls, including a magnetically induced velocity change (MIVC) method, a non-linear harmonics (NLH) method, x-ray diffraction, ultrasonic velocity measurements, and an a-c magnetic bridge.
The MIVC method depends on the change in ultrasonic velocity through steel as the magnetic field in the steel is changed. This method is not practical for in-line measurements, because it requires that the sensor be stationary on the surface of the steel part while the magnetic field is varying. An advantage of the MIVC method is that it can measure biaxial stress as an average value through the wall thickness as opposed to most other methods that respond only to the surface conditions. This method is capable of resolving the stress vector direction by making measurements in two orthogonal directions.
The NLH method has been used with some success to detect mechanical damage in pipelines. Since permeability and other magnetic properties change with both elastic and plastic stress and strain, the output of a NLH sensor will also change with these properties. The NLH method works in an operating pipeline at typical pig speeds, has reasonable sensitivity, and the data is not difficult to analyze. However, the method suffers from lift-off effects, requires a-c excitation, and requires complex instrumentation for operation.
The other methods for determining stress in steel parts, including x-ray diffraction, ultrasonic velocity measurements, and an a-c magnetic bridge require relatively complex instrumentation. Of these methods, only the magnetic bridge is suitable for in-line use, but its deployment for full pipe coverage requires many complex sensing circuits.
Means for determining locations of pipeline wall stress are well known in the relevant art and have been in use for at least 35 years. These means include various configurations of odometer wheels for determining locations of defects in a pipeline, as well as feature recognition by sensors in a pipeline pig that are able to detect pipeline characteristics, such as pipeline girth welds, weld spacing, valves, taps, and branch connectors that are often documented on pipeline maps. Odometer wheels of precisely known diameters are attached to a pig and roll on the inner pipe surface. The wheels may contain sensors, such as magnetic pulsers or optical encoders that produce data related to the wheel angular rotation. The data from these sensors and corresponding pipeline wall defect data are recorded by the pipeline pig. Subsequent playback of the data produces distance information (length of pipeline from launch point) correlated to the defect information. All inspection pig vendors use odometer wheels of similar design. The use of pipeline feature recognition and odometer wheels for distance measurement is common knowledge in the pipeline inspection business and all vendors have used these tools for many years.
Current methods for detection of mechanical damage in pipelines rely on secondary effects such as the effect of the strained area on magnetic flux leakage or the inference of plastic strain from deformation measurements. The Barkhausen method provides a more direct indication of plastically deformed regions of a pipe wall.